1. Field of the Invention
The present invention relates to an angular velocity sensor that is used to detect angular velocities in two-axis directions, for example.
2. Description of the Related Art
In general, a known angular velocity sensor includes a plurality of mass portions provided on a substrate. Japanese Unexamined Patent Application Publication No. 11-183179 and Japanese Unexamined Patent Application Publication No. 2006-138855 disclose an arrangement in which four mass portions are provided on the substrate along a circumference thereof and two adjacent mass portions in the circumferential direction are vibrated in directions opposite to each other. In such an arrangement, when an angular velocity about each of two axes is exerted on the substrate parallel to the surface of the substrate in a state in which the four mass portions are vibrated, the four mass portions are displaced in the vertical direction (i.e., the direction of thickness of the substrate) by the action of Coriolis forces. Thus, the angular velocity sensors disclosed in Japanese Unexamined Patent Application Publication No. 11-183179 and Japanese Unexamined Patent Application Publication No. 2006-138855 detect the angular velocities about the two axes by detecting the displacements of the four mass portions in the vertical direction.
Japanese Unexamined Patent Application Publication No. 2002-213962 discloses an arrangement in which a plurality of mass portions are arranged on a substrate side by side in the direction of a Y-axis and the plurality of mass portions are vibrated in the direction of an X-axis perpendicular to the Y-axis. The angular velocity sensor disclosed in Japanese Unexamined Patent Application Publication No. 2002-213962 detects an angular velocity about the Y-axis and an angular velocity about a Z-axis by detecting the displacements of the mass portions in the vertical direction of the substrate (i.e., the Z-axis direction) and in the Y-axis direction in a state where the plurality of mass portions are vibrated.
Japanese Unexamined Patent Application Publication No. 2002-509615 discloses an arrangement in which a substrate is disposed parallel to an XY-plane, a plurality of support legs each extending in a spiral shape from a support portion on the substrate are provided, and a ring-shaped vibrator is circumferentially provided at a distal end of each of the support legs. In such an arrangement, an angular velocity about an X-axis and an angular velocity about a Y-axis are detected by deforming and vibrating the ring-shaped vibrator in the XY-plane in a predetermined vibration mode and by detecting the displacement of the ring-shaped vibrator in the Z-axis direction (i.e., the vertical direction of the substrate).
The angular velocity sensors disclosed in Japanese Unexamined Patent Application Publication No. 11-183179 and Japanese Unexamined Patent Application Publication No. 2006-138855 are constructed so as to suppress the vibrations from leaking to the substrate by using the four mass portions in a combined manner. However, the amplitudes in the four mass portions need to be matched with each other in order to prevent a leak of the vibrations. In the angular velocity sensors disclosed in Japanese Unexamined Patent Application Publication No. 11-183179 and Japanese Unexamined Patent Application Publication No. 2006-138855, the four mass portions are supported so as to be able to vibrate using independent springs. Therefore, if manufacturing variations occur in the mass portions and the springs, a large difference in vibrations is generated among the mass portions when the mass portions are vibrated in a resonance state. This leads to a tendency for the vibrations of the mass portions to leak to the substrate, thus causing problems, such as fluctuations of an offset and an increase of noise, as the temperature changes. Further, because the four mass portions vibrate at different amplitudes, the Coriolis forces generated due to the angular velocities also differ from each other and the sensitivity varies between two modes of detections with respect to the two axes.
In the angular velocity sensors disclosed in Japanese Unexamined Patent Application Publication No. 11-183179 and Japanese Unexamined Patent Application Publication No. 2006-138855, particularly, the mass portions are supported in a state in which the springs are displaceable in both in the driving direction and the detecting direction. Therefore, coupling occurs between the vibration having been driven and the vibration to be detected, which increases the influence of a leak of the vibrations.
In the angular velocity sensor disclosed in Japanese Unexamined Patent Application Publication No. 2002-213962, a spring being subjected to the driven vibration and a spring being subjected to the detected vibration are provided independently of each another so that the coupling between the driven vibration and the detected vibration does not substantially occur. However, the disclosed angular velocity sensor is not suitable for specifications that are demanded in some practical applications because it does not detect the angular velocities about the two axes (i.e., the X-axis and the Y-axis) parallel to the substrate.
In the angular velocity sensor disclosed in Japanese Unexamined Patent Application Publication No. 2002-509615, a spring supports the ring-shaped vibrator in a state in which the spring is displaceable in all of the driving direction (i.e., the XY-planar direction) and the detecting direction (i.e., the Z-axis direction). Thus, because the spring and the ring-shaped vibrator are both used in driving and detecting vibrations, coupling is likely to occur between the driven vibration and the detected vibration.
Further, the ring-shaped vibrator is itself deformed in the XY-planar direction, and thus, the ring-shaped vibrator has a small width in the radial direction. The displacement of the ring-shaped vibrator in the Z-axis direction (i.e., the displacement caused by the Coriolis force) is detected by utilizing the electrostatic capacitance between a detection electrode provided on the substrate and the ring-shaped vibrator. However, because the width of the ring-shaped vibrator is relatively small, change in the capacitance between the ring-shaped vibrator and the detection electrode is also relatively small and the detection sensitivity tends to be reduced. In addition, the mass of the ring-shaped vibrator is relatively small such that the ring-shaped vibrator can be deformed in the XY-planar direction and the Z-axis direction. This causes another problem in that the resonance frequency of the ring-shaped vibrator is increased and the detection sensitivity of the angular velocity is reduced.